Difference between Primary Key and Unique Constraint in DB2

Primary Key and Unique Constraint are both used to enforce uniqueness in columns in a table in DB2, but there are some key differences between them:

1. Primary Key:
   • Unique identifier for each row in a table.
   • Can consist of one or multiple columns.
   • Automatically creates a unique index on the primary key columns.
   • By default, primary key columns cannot contain NULL values.
   • Only one primary key constraint is allowed per table.
2. Unique Constraint:
   • Ensures that all values in a column or a group of columns are unique.
   • Does not automatically create an index on the unique constraint columns.
   • Allows NULL values in the unique constraint columns, except for when the column is defined as NOT NULL.
   • Multiple unique constraints can be defined on a single table.

In summary, while both primary key and unique constraint enforce uniqueness in DB2, primary key is typically used to uniquely identify each row in a table, whereas unique constraint is used to enforce uniqueness on specific columns or groups of columns.